1. Field of the Invention
The present invention relates to the field of cleaning of a substrate surface and more particularly to the area of chemical and megasonic cleaning of a semiconductor wafer.
2. Discussion of Related Art
In semiconductor wafer substrate (wafer) cleaning, particle removal is essential. Particles can be removed by chemical means or by mechanical means. In current state of the art, particles are usually removed by both a combination of mechanical means and chemical means. The current state of the art is a batch process that places a number of wafers into a bath filled with a liquid and to apply high frequency (megasonic) irradiation to the liquid. Megasonic cleaning uses a ceramic piezoelectric crystal excited by a high-frequency AC voltage that causes the crystal to vibrate. The vibration causes sonic waves to travel through the liquid and provide the mechanical means to remove particles from the wafer surface. At the same time, chemicals in the liquid provide a slight surface etching and provide the right surface termination, such that once particles are dislodged from the surface by the combination of etch and mechanical action of the megasonics on the particles, these particles are not redeposited on the surface. In addition, chemicals are chosen such that an electrostatic repulsion exists between the surface termination of the wafer and the particles.
Until now, most megasonic irradiation has been applied to a bath in which the wafers are immersed. When using-a cleaning bath filled with a liquid to immerse the wafer in, it is necessary to immerse multiple wafers at the same time to be efficient. Single wafer cleaning is possible in a bath, but then the chemicals have to be reused, because of the volume of a single wafer bath.
So far, mechanical agitation in a single wafer cleaning method has been achieved in several ways. At first, when wafers are completely flat, brushes can be used to scrub the wafer surface. However, this method is not possible when the wafers have any topography (patterns) that can be damaged by the brushes. Moreover, the brushes don't reach in between the wafer patterns. Megasonic energy, which is the preferred mechanical agitation when patterns are present, can be applied to a liquid in a nozzle and this liquid can then be sprayed on the wafer. When spray methods are used in this way, the sonic pressure waves are confined to the droplets of the spray where they then lose a lot of their power. When the droplets hit the wafer surface, most of the remaining sonic energy is lost. Another method used is to apply megasonic pressure waves with a quartz rod suspended over the wafer surface with the cleaning solution building up between the rod and the wafer surface.
None of these attempts to apply megasonics to a single wafer surface is sufficiently efficient as they do not reduce the single wafer cleaning time enough, which is of the utmost importance. A single wafer cleaning approach should be much faster than a batch cleaning process in order to be competitive. Moreover, none of the current single wafer techniques are able to clean sufficiently both the front and the backside of the wafer at the same time. The only known technique to clean the front and backside at the same time is to immerse a batch of wafers in a bath and apply the acoustic waves from the sides of the wafers. In this manner, the acoustic waves travel parallel to the wafer surfaces to be cleaned. In silicon wafer cleaning, it is important to clean both sides of the wafer even though only the device side (front side) contains active devices. Contamination left on the device side can cause a malfunctioning device. Contamination left on the non-device side (backside) can cause a number of problems. Backside contamination can cause the photolithography step on the front side to be out of focus. Contamination on the backside can cause contamination of the processing tools, which in turn can be transferred to the front side of the wafer. Finally, metallic contamination on the backside, when deposited before a high temperature operation, can diffuse through the silicon wafer and end up on the device side of the wafer causing a malfunctioning of the device.
Polysilicon or amorphous silicon is deposited on a silicon wafer for different purposes. It can be the gate material of the transistor, or it can be used for local interconnects or it can be used as one of the capacitor plates in a capacitor structure. Most commonly, polysilicon or amorphous silicon is deposited on an insulating material, such as silicon dioxide. Polysilicon or amorphous silicon is usually deposited by a CVD (chemical vapor deposition) technique. The deposition of polysilicon or amorphous silicon usually occurs unselectively, that is, the entire wafer is covered with a layer of polysilicon or amorphous silicon. After such a blanket deposition, the wafers are covered with photoresist, the photoresist is exposed with UV light according to a certain designed pattern, and developed. Then the polysilicon or amorphous silicon is etched in a plasma reactor. The exposure of the photoresist determines the pattern in which the polysilicon or amorphous silicon will be etched. Usually, the polysilicon is used to conduct current from one place to another place or to collect charge as in a capacitor. In both cases, the dimensions are scaled down with every new generation of technology.
Until recently, dimensions not smaller than 0.3 μm (micron) were being used. However, technologies using poly-line dimensions smaller than 0.3 μm, such as 0.14 μm and even down to 0.1 μm are now being used. These poly-line dimensions and capacitor plate dimensions are so fragile a construction that they are prone to breakage. These constructs are so fragile that agitation may break them and cause a defective chip. After etching and photoresist removal, such as with an oxygen plasma (i.e. the ashing of the photoresist), the silicon wafers are usually riddled with particles. These particles have to be removed before going to the next device fabrication operation.
These particles are usually removed in a cleaning tool such as a wet bench. The particles are removed by immersing the wafers into a cleaning liquid and agitating the cleaning liquid with megasonic sound waves. This has worked well with poly-lines of 0.3 μm and above, however, when using poly-lines with dimensions smaller than 0.3 μm, megasonic sound agitation cannot be used as the megasonic sound agitation damages these fragile structures. Therefore, only chemicals can be used to clean particles when these fragile structures are exposed to the cleaning liquid. Although, even simple immersion into a cleaning liquid without agitation does remove some of the particles, it cannot remove all of the particles or even enough of the particles. Nevertheless, no alternative has existed and therefore, this is the only cleaning technique used on these fine structures.